Huntington's disease (HD) is one of nine fatal neurodegenerative disorders without effective treatment or cure, caused by an expanded CAG/polyglutamine repeat. The roles of the normal and mutant HD gene product, huntingtin remain uncertain. The objective of the proposed Mentored Clinical Scientist Development Award is to explore huntingtin mediated neurodegeneration. We propose to evaluate the roles of transcriptional dysregulation and protein processing in HD by pursuing the following Specific Aims: Specific Aim 1: Define the role of the NMDA receptor subunit NR2B, CBP, histone acetylation, and BDNF in aberrant neurite outgrowth and decreased survival of embryonic stem (ES) cells with expanded CAG repeats. Specific Aim 2: Identify transcripts involved in expanded polyglutamine-mediated transcriptional dysregulation. Specific Aim 3: Characterize the processing of polyglutamine containing proteins in undifferentiated and neuronally differentiated ES cells. We will pursue these Specific Aims utilizing an accurate, easily accessible, murine ES cell model. We find that neuronally differentiated ES cells with expanded CAG repeat domains develop features consistent with polyglutamine-mediated toxicity. Proposed experiments are expected to characterize full-length huntingtin protein processing and identify if the NMDA receptor subunit NR2B, CBP, histone acetylation, or BDNF are mediating neuronal dysfunction that precedes neurodegeneration in HD. Comparison of transcriptional profiles from neuronally differentiated ES cells with and without CAG repeats will be utilized to identify novel factors involved in HD pathogenesis. Once the pathologic mechanisms of mutant huntingtin are understood it will become possible to design rational therapeutics.